Effect of an array of small-scale Helmholtz resonators on turbulent boundary layer flow

Small-scale Helmholtz resonators embedded in a wall beneath a turbulent boundary layer can modify the energy of the turbulent velocity fluctuations when the resonator frequency is tuned in certain ways to the characteristic frequencies in the flow. This work experimentally investigates the impact of an array of small-scale Helmholtz resonators on a turbulent boundary layer, using particle image velocimetry and hot- wire anemometry. The specific aim of the study is to address whether the effect of a single Helmholtz resonator is amplified when multiple resonators are arranged in an array. In this investigation, we study a turbulent boundary layer flow at a friction Reynolds number of Reτ ≈ 2600 using two different array configurations. Both configurations consist of resonators with an orifice diameter of d^+ = 60, but differ in the streamwise spacing between the spanwise rows of resonators. The spacing was designed with an attempt to achieve a strong interaction between the resonator-induced velocity fluctuations and natural turbulence dynamics. Changes in the mean flow were observed downstream of both configurations. However, differences in energy distribution across scales were evident only in the configuration with a higher density of resonators. This arrangement also led to a reduction in the skin friction coefficient in the wake of the final rows of resonators, extending up to 1000l_ν beyond the last resonator. This indicates a possible accumulation of the effect of a single small-scale resonator, when using an array.